1. Field of the Invention
The invention relates to a gas turbine engine internal air system. In particular the invention concerns the control and distribution of internal cooling air in the guide vanes of a high pressure turbine section of a gas turbine propulsion engine.
2. Description of the Related Art
An engine internal air system has several important functions to perform for the safe and efficient operation of the engine. These include internal engine cooling, prevention of hot gas ingestion into the turbine disc cavities, cooling static and rotary components amongst others. Since an increasing amount of work is done on the air as it progresses through the compressor the air is usually taken as early as possible from the compressor commensurate with the requirements of each particular function. However in the case of cooling functions etc. in the region of a high pressure turbine the air is normally derived from a late stage of the compressor so that considerable work has been performed on it so it must be efficiently controlled and distributed in order to minimize performance losses.
In the HP turbine section cooling air is used to control the temperature of both static and rotative components, nozzle guide vanes and discs etc, by either cooling or heating them to ensure even temperature distribution to improve efficiency by controlling thermal growth. Thus blade tip and seal clearances may be maintained at optimum dimensions to maximize efficiency. High thermal efficiency requires high turbine entry temperature and heat conduction from the turbine blades into the turbine disc requires the disc to be cooled to prevent thermal fatigue and to control expansion and contraction rates and nozzle guide vanes must also be cooled internally. A particular problem arises in the HP turbine where in the latest generation of engines particular attention is paid to cooling the first stage disc and the immediately following nozzle guide vane annulus. These NGV""s incorporate trailing edge film cooling apertures so that internal distribution of cooling air inside the NGV is important as well as controlling the flow of air through the exit apertures into the gas path.
According to one aspect of the present invention a gas turbine engine internal air system including a multi-pass cooling arrangement for airfoil members in a non-rotary turbine stage wherein, in use, cooling flow from a source of cooling fluid flow is directed through a first passage in each of the airfoil members into a plenum adjacent a disc of a neighbouring rotary stage, and the airfoil members are each formed with a second internal cooling passage including an entry orifice communicating with the plenum and a number of exit apertures at least one of which receives cooling flow from the second cooling passage via fluid flow restricting means which restricts fluid flow through said aperture relative to at least one other of the exit apertures.